Method of making tin-base body solder and product

ABSTRACT

In the preferred embodiment, a method is presented for filling depressions in fabricated steel surfaces by applying a metallic body solder consisting of 15 to 20 weight percent copper, 2 to 3 weight percent zinc and the balance tin. The body solder alloy is heated to a temperature above about 210° C. to form a workable thixotropic paste that is applied with spreading onto the steel surface. Upon cooling, the body solder forms a dense, tightly adherent fill that is grindable and paintable in conjunction with the surrounding steel surface.

This is a Division of application Ser. No. 113,559, filed Jan. 21, 1980,now U.S. Pat. No. 4,248,905.

BACKGROUND OF THE INVENTION

This invention relates to a lead-free metallic body solder and, moreparticularly, to a tin-base body solder containing copper and zinc.

Body solder is traditionally a metal alloy adapted for application to afabricated steel or other metal surface to fill depressions therein andthereby form a smooth decorative surface. For example, in the automotiveindustry and elsewhere, body solder is used to fill a depressiondesigned about a welded seam or to repair a dent. Heretofore, alead-base alloy containing tin or antimony was used as a body solderbecause it was capable of being conveniently applied. When moderatelyheated, the alloy partially melts to form a workable paste that isreadily spreadable onto a steel surface using an oiled wooden paddle andhand applied pressure. The workable paste is formed over a widetemperature range, which substantially enhances its spreadability. Afterbeing applied and upon cooling, the alloy forms a dense, tightlyadherent fill having a surface adapted for finishing by grinding andprinting in conjunction with the surrounding steel surface. Although thelead-base alloy is generally adequate as a body solder, the use of leadrequires careful handling in the plant environment and is being reduced.

Therefore, it is an object of this invention to provide a metallic bodysolder requiring no lead content, which solder is adapted to be heatedand applied as a workable paste to suitably fill depressions in afabricated steel surface in a manner similar to traditional lead-basebody solders. The body solder exhibits good adhesion, good grindabilityand good paintability, comparable to the lead-base solders.

It is a more particular object of this invention to provide a bodysolder alloy composed predominantly of tin and containing copper andzinc, but no appreciable lead. The alloy forms a spreadable thixotropicpaste when heated to within a broad range of moderately elevatedtemperatures. The paste is suitable for spreading onto a steel surfacewithout heat damage and, upon cooling, bonds thereto. In addition, thesolder is capable of providing a paintable surface that resists saggingor flowing at typical curing temperatures.

It is also an object of this invention to provide a method of fillingdepressions in a fabricated steel surface prior to painting, whichmethod includes heating a tin-base copper-zinc alloy to within a broadrange of moderately elevated temperatures to form a spreadable paste.The paste is suitably applied to the depression without heat damage tothe steel and, upon cooling, bonds thereto. The filled surface issuitable for painting, in accordance with conventional steel surfacepainting practices, and remains intact during paint curing.

SUMMARY OF THE INVENTION

In the preferred embodiment, a method is provided for fillingdepressions in a fabricated steel surface comprising applying thereto,as a body solder, an alloy consisting of 15 to 20 weight percent copper(Cu), 2 to 3 weight percent zinc (Zn) and the balance tin (Sn) andcontaining no lead (Pb). The body solder alloy is applied as a workablethixotropic paste formed by heating to a temperature above about 210° C.Under these conditions, this specific family of alloys exhibits a pasteconsistency similar to molding clay or plaster. The paste comprisessufficient liquid to render the paste spreadable without excessiveforce, but there is not too much liquid which would permit the solderand liquid phases to separate, a condition known as eutectic separationor bleed. Although a spreadable paste is suitably formed at temperaturesas high as 385° C., the paste is preferable applied to the steel surfaceat about 270° C. or lower to avoid oxidation or other heat damage to thesteel.

The body solder alloy is preferably manufactured in a bar shape forconvenient handling and application. A homogeneous Sn-Cu-Zn melt is castand rapidly cooled to form a generally cylindrical ingot. The coolingrate is preferably between about 10° to 100° C. per second and resultsin a dendrite secondary arm spacing between about 1.0 to about 10microns. The ingot is thereafter extruded into the preferred bar shape.Extrusion is preferably carried out at about room temperature andinvolves a reduction in diameter of at least 4:1. It has been found thatrapid cooling and mechanical working of the cast alloy significantlyaffects its paste-forming properties so that, when heated, the extrudedbar more readily forms the desired spreadable thixotropic paste.

The fabricated steel surface is suitably prepared by cleaning, fluxingand tinning. Although the Sn-base alloy is self-tinning, the steelsurface is preferably tinned prior to filling to reduce the applicationtime and minimize the amount of alloy required. Tinning is suitablycarried out concurrent with fluxing by applying to the cleaned surface amixture containing a powder of the preferred Sn-Cu-Zn alloy and a zincchloride-type flux, heating to melt the alloy, and removing any excessby wiping. The body solder bar is selectively heated at one end to formthe paste, while being handled at the other. Formation of the paste isevidenced by a noticeable softening of the alloy. The paste is thenspread onto the tinned surface with a suitable utensil, such as a woodpaddle. After cooling, the body solder forms a dense, void-free fillthat tightly bonds to the adjacent steel. The surface is finished bygrinding. The claimed alloy has good feathering characteristics so thatthe edges of the fill surface merge smoothly with the surrounding steelsurface. The smooth fill surface is suitably painted in conjunction withthe surrounding steel surface and without special pretreatment. The bodysolder fill retains its shape even when heated to a temperature as highas about 200° C. during conventional curing of the paint coat.

The good spreading properties of the thixotropic paste are an importantfeature of this invention. The Sn-base alloy contains Cu and Zn inproportions suitable for forming the workable paste over an improvedbroad range of temperatures so that the paste remains workable for asuitable time after the heat source is removed. This temperature rangeincludes suitable low temperatures to enable filling without heat damageto the steel surface. The heated paste is suitably plastic for spreadingwithout hot shorting, a term of art referring to the tendency of a bodysolder material to fracture rather than flow during spreading. It is notunacceptably stiff or brittle. Also, the thixotropic paste retains thedesired shape after spreading and is not too liquid so as to flow or sagbefore solidifying. Furthermore, the paste acceptably adheres to thesteel surface without sticking to the preferred wooden applicator.

Thus, the Sn-Cu-Zn body solder alloy of this invention possesses asurprising combination of properties that render it particularlysuitable as a replacement for conventional Pb-base body solders. Forapplication, the Sn-Cu-Zn solder forms a spreadable paste at slightlyelevated temperatures suitable for steel surfaces. Upon solidifying, thealloy forms a dense, tightly adherent fill that is grindable andpaintable. Therefore, steel surfaces are suitably filled with theSn-base body solder by a method similar to the conventional method withthe Pb-base alloy. However, the preferred body solder alloy is formed ofSn, Cu and Zn, making it relatively safe for handling and use in atypical working environment.

DESCRIPTION OF THE DRAWINGS

The only FIGURE is a portion of the Sn-Cu-Zn tertiary compositionalgraph depicting Sn-base alloy compositions containing up to 40 weightpercent Zn and up to 40 weight percent Cu. Regions A-B-C-D and E-F-G-Hmarked in the FIGURE represent those alloy compositions suitable andpreferred, respectively, for body solder use in accordance with thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the preferred embodiment of this invention, aSn-Cu-Zn alloy was prepared for use as a body solder. The alloyconsisted of 82.5 weight percent Sn, 15 weight percent Cu and 2.5 weightpercent Zn. The metallic Sn was first heated in a clay-graphite cruciblewell above its melting point to about 550° C. To dissolve the highmelting Cu in the Sn melt, a flux comprising zinc chloride was added tothe crucible in a sufficient amount to completely cover the meltsurface. Thereafter, the required amount of solid Cu-metal was added tothe crucible. The flux improves the wetting of the solid Cu by the Snmelt and thereby substantially increases the dissolution of the Cu.After the Cu had completely dissolved, the zinc chloride flux wasskimmed from the melt surface and the required amount of metallic zincwas simply dissolved into the Sn-Cu melt.

The homogeneous Sn-Cu-Zn melt was cast into a water-cooled copper moldcomprising a cylindrical cavity that was 25 mm in diameter and 100 mm inlength. Water was circulated through the mold to rapidly cool the meltat a rate of about 25° C. per second. After cooling to room temperature,the cylindrical casting was extruded to form a bar having a diameter ofabout 6 mm and a length of about 400 mm.

The body solder was tested using a dented sheet steel panel. The cleandented surface was prepared by fluxing and tinning. The preferred fluxwas a commercially obtained acid zinc chloride material comprising, byweight, about 32 percent ZnCl₂, about 8.2 to 9.2 percent NH₄ Cl, about4.15 to 4.65 percent HCl, and the balance water and thickening agents inan amount to produce in a clear, colorless flux having a gel-likeconsistency. Although fluxing and tinning are suitably performed inseparate steps, it is preferred to carry out both in a single step byapplying a slurry comprising powder of the preferred Sn-Cu-Zn alloysuspended in the thick acid zinc chloride flux. At least 30 to 50percent of the powder was finer than 325 mesh, 20 to 40 percent wasbetween 200 to 325 mesh, 10 to 30 percent was between 100 to 200 meshand not more than 5 percent was greater than 100 mesh. The slurry wasapplied generously to the clean surface by brush and heated with adirect flame to cause the powdered alloy to melt and wet the steel. Theexcess was then removed by wiping with a suitable cloth.

While holding one end of the extruded body solder bar in one hand, thefree end was heated using an open natural gas flame, whereupon the alloyformed a spreadable paste. The formation of the paste was evidenced by anoticeable softening of the alloy, including a bending of the bar. Thepaste was thereafter applied to the fluxed and tinned dented surface byholding the bar near the surface and spreading the thixotropic pasteinto the dent with the aid of an oiled maple paddle.

After cooling, the Sn-base alloy formed a crack-free fill that tightlybonded to the adjacent steel. The fill surface was suitably finished bygrinding and displayed good feathering, which is the ability to grindthe fill progressively thinner at the edges. Feathering enables the fillsurface to be blended into the surrounding steel surface to create asmooth surface that is not discernable after painting. The smoothfill-steel surface was then painted in accordance with conventionalpractices to form a decorative finish. The paint was baked at atemperature of about 185° C. with no observable sagging of the bodysolder.

Good spreadability depends to some extent on the preferences of theparticular practitioner. However, the preferred alloy paste exhibitedmany properties generally desirable in body solder work. The pasteadhered well to the steel surface, but not appreciably to the oiledmaple paddle. Also, it exhibited a desired smooth consistency likemolding clay, in contrast to the gritty consistency like sand in water.This smooth consistency is obtained when liquifying produces many finesolid particles that are suitably suspended in the liquid phase, ratherthan fewer large particles that tend to precipitate. It has been foundthat the ability of the alloy to produce the smooth paste issubstantially improved by rapidly cooling and mechanically working thecasting. The thixotropic paste flowed in response to hand appliedpressure, and retained its shape thereafter. Neither hot shorting noreutectic bleed were observed.

A series of alloys were prepared for testing in accordance with theaforementioned preferred procedure to determine the effect of variationof the alloy composition on the formation of a spreadable paste. Insummary, alloys containing less than about 10 weight percent Cu formedexcessive liquid phases and were difficult to handle. An excessiveliquid phase was indicated by eutectic bleeding or on-panel running ofthe paste. Alloys containing greater than about 25 weight percent copperevidenced unacceptable brittleness in the paste that resulted in hotshorting. Hot shorting is typically found in pastes wherein theproportion of the solid phase to the liquid phase is too large. Sn-Cualloys containing no zinc were difficult to work and frequently squeakedduring application, similar to the squeaking produced when Sn metal isdeformed at room temperature. Alloys containing less than about 1.0weight percent zinc formed a paste over an extremely limited temperaturerange so that it was extremely difficult to control the paste propertiesduring application. Alloys containing greater than about 7.5 weightpercent Zn also exhibited unacceptable hot shorting. These results areshown graphically in the FIGURE as the region A-B-C-D. Alloys havingcompositions falling with the region A-B-C-D formed suitable thixotropicpastes for body solder use.

Within the region A-B-C-D, alloys having compositions falling withinregion E-F-G-H formed preferred body solder pastes. Alloys within thepreferred region were less sticky as indicated by a reduced tendency tocling to the maple paddle. Also, the alloys were less stiff andevidenced a reduced resistance to plastic flow as a result of theapplicator force. More particularly, these preferred alloys comprise 15to 20 weight percent Cu, 2 to 3 weight percent Zn and the balance Sn.The composition of the body solder alloy represented at point J anddescribed in the preferred embodiment has been found to serve as anexcellent body solder when applied at a temperature in the range ofabout 210° C. to 270° C.

Equally important to the spreading properties is the temperature atwhich the workable paste is formed. The Sn-Cu-Zn body solder melts overa range of temperatures, so that liquid and solid phases coexist in amushy mixture between the solidus and liquidus temperatures, as istypical of non-eutectic metal alloys. It is theorized that a workablepaste comprises about 25 to 50% liquid on a weight basis and that thesepoints, rather than the solidus and liquidus temperatures, determine theuseful temperature range of the body solder alloys. The usefultemperature range was experimentally determined for the preferred alloyconsisting of 15% Cu, 2.5% Zn and the balance Sn. In general, the alloyformed a spreadable paste at about 210° C. and continued to bespreadable up to a temperature of about 385° C. or higher. In view ofthe subjective nature of the spreadability property, tne extremetemperatures represent generalizations and are not intended as a preciselimit. However, in general, it has been found that Sn-Cu-Zn alloyspreferred for body solder use; i.e., alloys falling in the E-F-G-Hregion of the FIGURE, become spreadable when heated to near 210° C.,which minimum is only slightly affected by differences in thecomposition.

In the typical practice, the alloy is heated to form the paste and coolswhen applied and spread. Steel surfaces are damaged when heated to about270° C., primarily because of oxidation or distortion. However, sincethe paste is spreadable at temperatures as low as 210° C., iteffectively provides a 60° working range. This range is sufficientlybroad to enable the practitioner to apply the paste without heat damageto the surface, yet have a reasonable time for spreading. In instanceswhere cooling causes the alloy to stiffen before spreading is complete,the alloy may be carefully reheated on the surface, for example, with anopen flame, to reform the paste in situ without heat damage to thesurface. It is noted that the body solder alloys of this inventiontypically form pastes at temperatures up to about 385° C. or higher. Inappropriate circumstances, the alloy may be heated to a temperatureabove steel-damaging and cooled after removal from the heat source to asuitably low temperature for application. Thus, the body solder alloysform spreadable pastes over an improved broad temperature range, whichincludes a significant portion lying below the steel-damagingtemperature.

Another feature of the preferred body solder alloys of this invention isthat they retain the desired shape during painting and, moreparticularly, during baking of the paint coat. Painted steel surfacesare typically baked up to about 200° C. Thus, although the preferredSn-Cu-Zn alloys are spreadable at 210° C., they are not so soft at about200° C. that they sag or flow during paint baking.

In the preferred embodiment, the body solder consisted of Sn, Cu and Znmetals. In the usual practice, the metals are commercially obtained andtypically contain impurities, including Pb and arsenic As. Thus, thebody solder alloys also typically include these impurities. Preferably,the body solder alloy does not contain more than about 0.1 weightpercent Pb, 0.07 weight percent As and 0.1 weight percent otherimpurities.

Although in the preferred embodiment the body solder was shaped into abar and selectively heated to form the spreadable paste, it is apparentthat the alloy is suitable for application by other techniques, such asfrom a mush pot, and that its usefulness is not limited to a particularshape. When applying the body solder as a bar, it has been found thatrapidly cooling and mechanically working the cast alloy substantiallyenhances its ability to readily form the paste upon heating. Therefore,for bar application, the casting is cooled at a rate faster than about1° C. per second and preferably at a rate between about 10 to about 100°C. per second. Suitable cooling is evidenced in the alloy microstructureby a dendrite secondary arm spacing of between about 1 to about 10microns, although the connection between the dendrite structure andpaste formation is not certain. The casting is then suitably extruded ata temperature less than about 150° to effect a reduction in diameter ofat least 4:1. Alternately, the bar is suitably formed by rolling thecasting to subject the alloy to the desired mechanical work.

Although this invention has been described in terms of certainembodiments thereof, it is not intended that it be limited to the abovedescription but rather only to the extent set forth in the claims thatfollow.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method ofmanufacturing a tin-base alloy suitable for use in filling a depressionin a fabricated steel surface, said tin-base alloy consistingessentially of about 15 to 20 weight percent copper, about 2 to 3 weightpercent zinc and the balance tin, said method comprisingcasting ahomogeneous melt of the alloy, cooling said casting at a rate sufficientto form in the alloy microstructure a dendrite secondary arm spacingbetween about 1 to 10 microns, and working said casting to an extentequivalent to reducing a cross-sectional diameter by at least 4:1.
 2. Amethod of manufacturing a tin-base alloy suitable for use in filling adepression in a fabricated steel surface, said tin-base alloy consistingessentially of about 10 to 25 weight percent copper, about 1 to 7.5weight percent zinc and the balance tin, said method comprisingcasting ahomogeneous melt of the alloy into a suitable shape adapted forextrusion, cooling said casting at a rate faster than about 1° C. persecond, and extruding said cast shape to form a bar, said alloy beingworked during extrusion such that the alloy readily forms a spreadablepaste upon heating to above 210° C.
 3. A method of manufacturing atin-base alloy suitable for use in filling a depression in a fabricatedsteel surface, said tin-base alloy consisting essentially of about 15 to20 weight percent copper, about 2 to 3 weight percent zinc and thebalance tin, said method comprisingcasting a homogeneous melt of thealloy into a suitable shape adapted for extrusion, cooling said castingat a rate between about 10° to about 100° C. per second, and extrudingsaid cast shape at a temperature less than about 150° C. to form a bar,said alloy being worked during extrusion to an extent equivalent toreducing a cross-sectional diameter by at least 4:1.
 4. A cast andextruded bar comprising an alloy suitable for use as a body solder tofill a depression in a steel surface, said alloy consisting essentiallyof, by weight, about 10 to 25 percent copper, about 1 to 7.5 percentzinc and the balance tin, said bar formed by casting a homogeneous meltof the alloy, rapidly cooling and extruding the cast and cooled alloysuch that the product alloy forms a workable thixotropic paste uponheating.
 5. A cast and extruded bar comprising an alloy suitable for useas a body solder to fill a depression in a steel surface, said alloyconsisting essentially of, by weight, about 15 to 20 percent copper,about 2 to 3 percent zinc and the balance tin, said alloy bar formed bycasting a homogeneous melt of the alloy into a suitable shape forextrusion, cooling the casting at a rate faster than about 1° C. persecond and extruding the cast shape at a temperature less than about150° C. under conditions suitable for working the alloy to an extentequivalent to reducing a cross-sectional diameter by at least 4:1.